Homogenizing and/or dispersing device comprising endless screws

ABSTRACT

An apparatus for homogenizing and/or dispersing at least one viscous substance and at least one solid and/or at least one other substance of different viscosity has in a space ( 3 ) in the housing ( 1 ) axis-parallel rotatable shafts ( 4 ) disposed along a circle with equal central-angle distance and provided with tightly intermeshing screw elements ( 6 ). The shafts ( 4 ) in the area outside the openings ( 9, 11, 12 ) in the housing ( 1 ) are provided with screw elements ( 6 ) having a pitch (S) of at most three quarters of the outside screw diameter (D) over a length (L=L 1 +L 2 +L 3 ) corresponding to at least ten times the outside diameter (D) of the screw elements ( 6 ).

[0001] This invention relates to an apparatus for homogenizing and/ordispersing at least one viscous substance and at least one solid and/orat least one other substance of different viscosity according to thegeneric part of claim 1.

[0002] Screw extruders have the disadvantage of frequently obtainingpoorer qualitative results when homogenizing, kneading and dispersingsolids in a viscous matrix or homogenizing substances of very differentviscosity than with batchwise kneading. This also applies to twin screwextruders. To improve homogenization, the two shafts of twin screwextruders are therefore generally provided not only with screw elementsbut also with rotationally fast so-called kneading blocks which consistof two or more disk cams mutually offset at an angle in thecircumferential direction and with a contour formed of a plurality ofcircular arcs. Such kneading blocks are described for example in DE 813154 B and EP 0 422 272 B1.

[0003] Further, EP 0 788 867 B1 discloses an apparatus for continuousprocessing of flowable materials according to the generic part of claim1. In this apparatus, homogenization is also effected substantially bysuch kneading blocks. Kneading blocks have the disadvantage, however,that low energies are conducted into the product to be processed in thedeep channel center while very high energies are conducted in thechannel tapering in a wedge shape to the housing, which can lead notonly to a specifically high energy consumption but also to partialoverstressing of the product and high wear in the area of the kneadingblocks and housing. Moreover, the axially open construction of thekneading blocks can cause product fractions to pass through the kneadingblocks easily on a short path and without energy consumption, i.e.without homogenization.

[0004] DE 28 54 207 B2 discloses a screw extruder having a mixing zonewith a length of 4-12 D (D=outside diameter of a screw element), thepitches being between 0.5 to 2 D in the feed zone and 0.3-1 D in thedischarge zone.

[0005] The problem of the invention is to state the requirements for thedesign of a generic apparatus for perfect homogenization and/ordispersion.

[0006] This is obtained according to the invention by the apparatuscharacterized in claim 1. The subclaims render advantageous embodimentsof the invention.

[0007] The inventive apparatus has in a space between a housing and aninside core along a circle with equal central-angle distance at leastthree axis-parallel rotatable shafts provided at least partly withtightly intermeshing screw elements. The housing is provided on itsinside and the inside core on its outside with axis-parallel concavecircular segments that receive the particular shaft with the screwelements.

[0008] The circle on which the shafts are disposed is a circular arc orcan be a full circle, so that the process space can be disposed aroundthe circular arc or is annular. The housing and/or inside core isprovided with openings, for example material supply openings and/ormaterial and/or gas outlet openings.

[0009] Since the screw elements in such an apparatus have a distinctlyreduced homogenizing or dispersing effect in the area where the housingand/or inside core has openings, the axial area located outside saidopenings must have a length corresponding to at least ten times,preferably fifteen times, the outside diameter of the screw elementsaccording to the invention.

[0010] This length also relates only to the area where the viscoussubstance is present in a 100% melted or liquid state. That is, thescrew length necessary for melting is not included in the inventiveprocess part.

[0011] The same holds when substances of different viscosity arehomogenized with the inventive apparatus and at least one of saidsubstances must first be melted in the apparatus. Then, too, the lengthof the screw elements on the shafts outside the openings provided in thehousing and/or inside core must be at least ten times, preferablyfifteen times, the outside screw diameter, namely in the area where allsubstances are liquid or melted.

[0012] The length where the solids content in the liquid, i.e. possiblymelted, substance or, upon homogenization of substances of differentviscosity, all substances to be homogenized are present in a liquid,i.e. possibly melted, state in the apparatus and that is located outsidesaid openings can also be designated the process length. That is, theshafts are provided with screw elements over a process lengthcorresponding to at least ten times, preferably at least fifteen times,the outside screw diameter according to the invention.

[0013] Moreover, the screw elements of which the process length iscomposed have a pitch of at most three quarters, preferably at most twothirds and especially preferably at most half, the outside screwdiameter over a length corresponding to at least three times, preferablyat least five times, the outside screw diameter in order to obtain thedesired dispersion and homogenization.

[0014] When these conditions are met, solids in a viscous matrix can bedispersed or homogenized continuously with the inventive apparatus withsuch an outstanding result as could hitherto be attained only batchwisewith kneaders. The same applies to the homogenization of substances ofdifferent viscosity.

[0015] This is because under these conditions the number of zones whereadjacent screw elements intermesh, i.e. the lands or flights of adjacentscrew elements overlap, is multiplied.

[0016] In a twin screw extruder the number of transitions where theconcave circular segments on one and the other side of the housingintersect on a housing cross section is altogether two, in an apparatusaccording to EP 0 788 867 B1 with three shafts the number of transitionswhere the concave circular segments on the inside of the housing and theoutside of the inside core intersect is altogether four, and with twelveshafts in a closed circle altogether twenty-four.

[0017] In strictly mechanically terms, at one screw revolutiontwenty-four wedges are passed with the twelve shafts and two in the twinscrew. If twenty turns with pitch=1×diameter are disposed over thepartial process length of 20 Ø in each case, the number of passages perrevolution increases to 20×24=480 with 12 shafts and to 2×20=40 in atwin screw.

[0018] In a three-shaft machine four wedges are simultaneouslyeffective, which means that four wedges become effective per screwrevolution, i.e. eighty wedge passages in case of twenty turns.Analogously the results are as follows at smaller pitches for a: Twinscrew pitch ¾ Ø 53.3 wedge passages  3 shafts 106.7 wedge passages 12shafts 640 wedge passages Twin screw with pitch ⅔ Ø 60 wedge passages  3shafts 120 wedge passages 12 shafts 720 wedge passages Twin screw withpitch ½ Ø 80 wedge passages  3 shafts 160 wedge passages 12 shafts 960wedge passages

[0019] where Ø is the outside screw diameter.

[0020] The effect of the intermeshing zones on the dispersion andhomogenization of one or more solids or the other substance fractions ina viscous matrix is based substantially on the braking influence of thehousing being substantially eliminated when material flows through theintermeshing zone, which simultaneously involves an increase in materialspeeds, and results in an extension of the involved material. Further,when material hits the outside diameter of the receiving screw flightthat is rotating at circumferential speed, the material is removed fromthe screw surface of the delivering screw, reoriented and directed intothe contracted cross section of the receiving screw with a new flowdirection, as in plowing. Via the viscosity of the continuous phase,this causes traction and bending forces to be exerted on the moistenedsolid agglomerates, which can then break. Since the amount of stress islimited by the viscosity, single break and/or repeated break must takeplace with the passage of many tandem-mounted intermeshing zones overthe duration of stressing. The screws act as a strict conveying andpressure buildup system over the circumferential length fromintermeshing zone to intermeshing zone, as in a conventionalsingle-shaft extruder.

[0021] That these processes play an essential part for homogenizationand dispersion of the solid is to be inferred from the fact that evenwith solid particles having a particle size in the micron range in aviscous matrix the inventive apparatus obtains perfect dispersion andhomogenization, which is in no way inferior to batchwise dispersion andhomogenization in a kneader.

[0022] The same result can be achieved when homogenizing substances ofdifferent viscosity with the inventive apparatus.

[0023] The screws of the inventive apparatus are preferably of single-or double-flighted design. Single-flighted screw elements are especiallypreferred because the flight width can be selected to be large comparedto the channel width, i.e. a high cross-sectional contraction of thescrew channel can be obtained.

[0024] The screw profile of the inventive apparatus preferably consistsof a plurality of circular arcs, whereby on the outside and inside theparticular radius constitutes the curvature and for the flankstherebetween the center distance determines as the radius the maximumcurvature.

Ax=D/2+d/2

[0025] Ax Center distance of two screws

[0026] D Outside diameter

[0027] d Core diameter

GZ(2alpha+beta+gamma)=360°

[0028] GZ Number of channels

[0029] alpha Flank angle

[0030] beta Core angle

[0031] gamma Flight angle

[0032] In the standard version, beta=gamma is selected.

[0033] The maximum possible relations for D:d result from theabovementioned mathematical relations.

[0034] In the inventive apparatus a comminution of the solid particleagglomerates is thus effected in the transfer process of material fromthe delivering to the receiving screw through extension and bending(reorientation) of the viscous matrix, whereby the amount of stress towhich the material is exposed can be influenced substantially by thenumber of intermeshing zones, the duration of stressing and via theflight width, the process engineering requirements can be influencedvery finely by the mechanical variables.

[0035] Through the large screw surface involved in the processingoperation and the small quantity of material in relation thereto, nopartial overheating or other overloading of the material is to beexpected in the inventive apparatus, but rather a uniform load in theintermeshing zones. Thus also results in uniform energy conversionthroughout the material to be processed along with low wear of thescrews. In addition there is the crucial advantage that axial bypassingof parts of the material stream is prevented, i.e. no escape ispossible.

[0036] The inventive apparatus can be used to produce for examplenonconductive plastics by dispersing and homogenizing carbon black witha quality that was hitherto hardly attainable with kneaders. Further, itcan be used to homogenize and/or disperse or break up paints, fillersand foods such as chocolate. For example, it can produce the startingmaterial required for spinning microfibers, i.e. a spinnable plastic towhich coloring pigments or other solids are added in the micron or nanorange. Said microfibers have a diameter that is only slightly greaterthan the diameter of the coloring pigments or other solid particles. Ifthe solid aggregates are not finely dispersed and homogenized, thespinning jets will become clogged or the fibers tear during spinning.Further, the inventive apparatus can be used for example to produce thematerial for plastic foils or protective paints. Since the inventiveapparatus is also suitable for homogenizing plastics with very differentpolymer chain lengths, it can also be used for example to modify bi- ormultimodal plastics.

[0037] The shafts of the inventive apparatus can be driven in the samedirection but can also be formed to turn in opposite directions.

[0038] In the following, an embodiment of the inventive apparatus willbe described in more detail with reference to the drawing, in which:

[0039]FIG. 1 shows a longitudinal section through the apparatus;

[0040]FIG. 2 shows a cross section of the apparatus along line II-II inFIG. 1;

[0041]FIG. 3 shows a cross section through a further embodiment of theapparatus with three shafts with intermeshing screw elements in acircular arc-shaped screw space in the housing;

[0042]FIG. 4 shows a view corresponding to FIG. 3 without the housing;

[0043]FIG. 5 shows a view corresponding to FIG. 4 but with threesingle-flighted screw elements;

[0044]FIGS. 6 and 7 show a perspective view and plan view of the threescrew elements according to FIG. 5; and

[0045]FIG. 8 shows a side view of the screw shafts corresponding to FIG.1 but with single-flighted screw elements having flight widths ofdifferent size in different axial areas.

[0046] According to FIGS. 1 and 2, the apparatus has housing 1 and axialinside core 2. Between housing 1 and inside core 2 there is annularscrew space 3 with a plurality of axis-parallel disposed shafts 4.

[0047] Housing 1 is closed on the input side by end-plate 5 throughwhich shafts 4 extend that are driven in the same direction or oppositedirections by a gear (not shown). A plurality of double-flighted screwelements 6 are disposed rotationally fast on each shaft 4.

[0048] As indicated by FIG. 2, screw elements 6 of adjacent shafts 4intermesh with little play, i.e. largely tightly.

[0049] Housing 1 is provided on the inside, and core 2 on its outside,with axis-parallel concave circular segments 7, 8. In cross section theinside of housing 1 and inside core 2 are thus formed in a rosette shapeof concentric circular segments 7 and 8 whose center is located in thecylinder surface of the circle on which the axles of shafts 4 arelocated. Screw elements 6 mesh with circular segments 7, 8 with littleplay, i.e. largely tightly.

[0050] At the input-side end facing end-plate 5, housing 1 has opening 9on the top through which material to be processed (not shown) issupplied to screw space 3.

[0051] Gases emitted by the material flow outside through openings 11,12 in housing 1. Said openings can e.g. also be provided in inside core2 if it is of hollow design.

[0052] Shafts 4 extend in the area located outside openings 9, 11, 12over a length corresponding e.g. to twenty times outside diameter D ofscrew elements 6.

[0053] According to FIGS. 2, 3 and 4, screw elements 6 have anelliptical contour in cross section having in the area of arcs A, B andA′, B′ a curvature corresponding to D/2, and in area E, F and E′, F′ acurvature corresponding to A_(x)-½ D, where A_(x) signifies the axialdistance between two shafts. Angles α, β and γ, the curvatures of theareas between B and E, F and B′ and A, E′ and F′, A′ can be determinedby well-known methods of analytic geometry, as described in EP 0 002 131B1.

[0054] According to FIG. 2, the apparatus has through the twelve shafts4 altogether 24 wedges 13, 14 or transitions where concave circularsegments 7 on the inside of housing 1 and concave circular segments 8 onthe outside of screw space 3 intersect. Between the two transitions 13,14 there are zones 15 where screw elements 6 of adjacent shafts 4intermesh tightly. At a pitch of ½ D altogether 960 intermeshing zones15 are thus formed on screw length L composed of screw lengths L1between material supply opening 9 and degassing opening 11, screw lengthL2 between the two degassing openings 11 and 12, and screw length L3between degassing opening 12 and the material-discharge end of shafts 4,with L=20 D.

[0055] The effect of intermeshing zones 15 on the dispersion andhomogenization of a solid or the other substance fractions in a viscousmatrix is explained in more detail with reference to FIGS. 5 to 7, whichshow three intermeshing single-flighted screw elements with pitch S. Thescrew conveying direction is rendered by arrow 10. For the(right-handed) screws to convey according to arrow 10, they have thedirection of rotation shown in FIG. 7.

[0056] Material moving in the direction of arrow 16 is exposed to thebraking influence of housing 1 in the area of chamber 18 formed byconcave circular segments 7 and 8 (FIG. 3). The effect of intermeshingzones 15 on the dispersion and homogenization of a solid or the othersubstance fractions in a viscous matrix is based substantially on saidbraking influence of housing 1 being substantially eliminated whenmaterial flows through intermeshing zones 15, which simultaneouslyinvolves an increase in material speeds and results in an extension ofthe involved material. Further, when material hits the outside diameterof flight K of receiving screw 6 _(auf), said flight rotating atcircumferential speed, the material is removed from the screw surface ofdelivering screw 6 _(ab), reoriented and directed into the contractedcross section at 17 with a new flow direction, as in plowing. Via theviscosity of the continuous phase, this causes traction and bendingforces to be exerted on the moistened solid agglomerates contained inthe matrix so that they can break.

[0057]FIG. 7 clearly indicates chamber 18, receiving side 6 _(auf) beingshown at the top and delivering side 6 _(ab) at the bottom, with maximumpossible flight width B. If B is made smaller, the receiving anddelivering traversing surfaces simultaneously become larger and thetransfer resistance thus smaller.

[0058] By changing flight width B one can thus influence the function.At an increase of pitch S, e.g. at twice the pitch, an increased ordouble throughput is obtained.

[0059] According to FIG. 8, single-flighted screw elements 6 havegreater flight width K in axial area M of the apparatus than screwelements 6 in axial area N thereof. Screw elements 6 with greater flightwidth K can be disposed on the material output side and screw elements 6with smaller flight width K on the material input side. However, it isalso possible to dispose the screw elements with the greater flightwidth on the material input side and screw elements 6 with the smallerflight width on the material output side. Also, it is possible toprovide axial areas with screw elements with greater flight width andaxial areas with screw elements with small flight width alternately.

[0060] The flow resistance in the intermeshing zones can be controlledby the different flight width. The greater the flight width, the greaterthe resistance upon entering said zones.

EXAMPLE

[0061] An inventive apparatus with twelve shafts and a melt-filledprocess length of 23 D (D=outside diameter of screw elements), of which10.5 D process length was equipped with a screw of ⅔ D pitch or less,was used to homogenize a polymer with 25 wt % carbon black. The formedpellets were subjected to optical measurement. An absolute dispersionwas ascertained, at a particle size of <5 microns. This quality couldhitherto be obtained only in a multistage method on a batch kneader.

1. An apparatus for homogenizing and/or dispersing at least one viscoussubstance and at least one solid and/or at least one other substance ofdifferent viscosity, having at least three axis-parallel rotatableshafts disposed in a screw space in a housing along a circle with equalcentral-angle distance and provided at least partly with tightlyintermeshing screw elements, the housing being provided on the radiallyexternal and internal sides of the screw space with axis-parallelconcave circular segments receiving the particular shaft and withopenings into the screw space, characterized in that the shafts (4) areprovided in the area outside the openings (9, 11, 12) over a length(L=L1+L2 +L3) corresponding to at least ten times the outside diameter(D) of the screw elements (6) with screw elements (6) having a pitch (S)of at most three quarters of the outside screw diameter (D) over alength of at least three times the outside diameter (D) of the screwelements (6).
 2. An apparatus according to claim 1, characterized inthat the pitch (S) amounts to at most two thirds of the outside screwdiameter (D).
 3. An apparatus according to claim 1 or 2, characterizedin that the screw elements (6) are of single-flighted design.
 4. Anapparatus according to any of the above claims, characterized in thatthe screw elements (6) have an elliptical contour in cross section, thecurvature in the area of the major axis (A, B, A′, B′) corresponding tohalf the outside screw diameter (D) and in the area of the minorvertices (E, F, E′, F′) to the axial distance (Ax) of two shafts (4)minus half the outside screw diameter (D).
 5. An apparatus according toclaim 3, characterized in that the flight width (K) of the screwelements (6) is different in individual axial areas (M, N) at constantpitch (S).